Method for the microbiological decontamination of soil

ABSTRACT

The present invention relates to a method for the microbiological decontamination of soil, in which 
     1) the contaminated soil is excavated and treated, 
     2) the treated soil is banked up on a prepared subsoil in the form of regeneration clamps, and 
     3) the clamps are provided with oxygen while microbiological degradation of the contaminants takes place. 
     The method comprises mixing the contaminated soil with, preferably contaminated, concrete and/or building rubble, before the mixture is banked up in the form of regeneration clamps, the concrete and/or building rubble having been comminuted before mixing to a particle size of ≦10 mm, preferably ≦6 mm. 
     The method according to the invention makes possible, within a short time, decontamination even of soils whose permeability for air and water is only low, and also of soils where the distribution of contaminants is highly inhomogeneous, or where the concentration of contaminants is very high. 
     Particularly good results are achieved when the clamps are additionally sprinkled with treated water from the operator&#39;s own treatment plant.

The present invention relates to a method for the microbiologicaldecontamination of soil, in which

1) the contaminated soil is excavated and treated,

2) the treated soil is banked up on a prepared subsoil in the form ofregeneration clamps, and

3) the clamps are provided with oxygen while microbiological degradationof the contaminants takes place.

Decades of industrial utilization of terrain resulted in pollution ofthe soil, for example caused by inappropriate deposition of residues andleakages from tank systems, pipeline systems and pump systems. Suchcontaminations require a treatment if they adversely influence thegroundwater or when polluted soil must be excavated for buildingpurposes.

The contaminants which lead to soil contamination can belong to a wideclass of substances and can have very different origins. Examples ofpotential contaminants include mineral oils of all processing stages(degrees of refining), such as crude oils, diesel oils, fuel oils,gasolines, industrial oils, furthermore chlorohydrocarbons, such astrichloroethylene, tetrachloroethylene, trichloroethane anddichloromethane, organic solvents, such as, for example, phenols,alcohols, aromatic hydrocarbons, aldehydes, acids, esters, ketones andethers, but also various plastics, various other organic and inorganicsubstances and, last but not least, also pesticides and herbicides.

Many procedures are possible for the sanitation of contaminated soils.For example, there are the so-called "on-site" methods, in which thecontaminated soil is first removed. In a different location, thecontaminants are then removed by means of various treatment methods,such as, for example, thermally, chemically, microbiologically or bymeans of mechanical washing, and the purified soil is used to fill upthe cavities formed by removal of the soil.

A method which has proved economical is the microbiological "on-site"degradation of harmful organic substances. The desired end products ofthe aerobic degradation of organic substances are carbon dioxide andwater. A decisive factor for optimum removal of harmful organicsubstances is the adjustment of so-called environmental factors--suchas, for example, oxygen content, pH, moisture and the presence ofsufficient amounts .of nutrients for the microorganisms. Other factors,such as, for example, the solubility of the harmful substances in water,are also highly important.

Since there is such a multitude of factors, many different methods formicrobiological "on-site" degradation of harmful organic substances havebeen described (cf., for example, Chem.-Ing.-Tech. 59 (1987), No. 6,pages 457-464). For example, in the Shell BIOREG method (cf.Chem-Ing.-Tech. 59 (1987), No. 6, page 461, right-hand column), theexcavated contaminated soil is mixed with ground pine bark on a preparedsubsoil and banked up in the form of regeneration clamps (height: 1.2m). It is also possible to additionally incorporate aeration layersequipped with drainage pipes.

In another method (cf. Chem.-Ing.-Tech. 59 (1987), No. 6, page 461,right-hand column), contaminated soil is mixed with organic material(for example straw) and then inoculated with fungi causing white rot inorder to break down polycyclic aromatic hydrocarbons. The fungi possessan enzyme system which is capable of breaking down cellulose and alsosuitable for breaking down the polycyclic aromatic hydrocarbons.

Finally, an experiment carried out by Deurag is also known (cf.Wirtschaftswoche No. 37 of Sep. 9, 1988, pages 101 and 102) in which thecontaminated soil is first simply spread and waste water from thecompany's own sewage plant which contained bacteria was sprinkled ontothe soil. After this, all that happened was that the soil was turnedover twice per year and time was allowed to pass (principle of "landfarming").

A great disadvantage of the known biological soil decontaminationmethods is the limited applicability with regard to the nature of thesoil. For example, silty clay materials are not accessible to amicrobiological sanitation because of their low permeability to air andwater. Other grave disadvantages of the known "on-site" methods whichwork with microbiological purification of the soils are, in particular,the very slow degradation of the contaminant--the durations of thetreatments described in the literature are of up to 4 years--as well asthe occurrence of problems in the case of soils where the concentrationof pollutants is very high (i.e. about >2000 mg/kg), or in the case ofsoils where distribution of the individual pollutants is highlyinhomogeneous. Moreover, methods in which the contaminated soil is mixedwith tree bark or chopped straw for sanitation purposes have thedisadvantage that the bearing capacities of such soils are greatlyreduced and the soil is therefore unsuitable as foundation soil.

It was therefore the object of the present invention to provide a methodfor the microbiological decontamination of soil, in which it is possibleto sanitate even polluted soils which could be regenerated only withdifficulty or not at all using conventional microbiological methods,while keeping the treatment time as short as possible. For example, themethods should make it possible to decontaminate in particular alsofinely-particulate materials whose permeability for air and water isonly low, for example silty clay materials, and also soils where thedistribution of the pollutants is highly inhomogeneous, or those wherethe concentration of the pollutants is very high.

The object is achieved according to the invention by a method for themicrobiological decontamination of soil, in which

1) the contaminated soil is excavated and treated,

2) the treated soil is banked up on a prepared subsoil in the form ofregeneration clamps, and

3) the clamps are provided with oxygen while microbiological degradationof the contaminants takes place.

The process comprises a treatment of the contaminated soil, in which thesoil is mixed with concrete and/or building rubble which has previouslybeen comminuted to a particle size of<10 mm, preferably≦6 mm.

It was surprising and not predictable that in the method according tothe invention the concentration of harmful substances in contaminatedsoils is reduced drastically after an extremely short period of, ingeneral, less than 1 year, even only 1/2 year, so that these soils canbe used for filling the cavities resulting from the removal of soil. Dueto the firmness, these soils are also suitable as a foundation subsoil.Another important advantage of the method according to the invention isthe fact that even finely-particulate soils whose permeability for airand water is only low, such as, for example, silty clay materials, aswell as soils whose concentration of pollutants is high and/or where thedistribution of pollutants is inhomogeneous, can be decontaminated withoutstanding results.

The sanitation method according to the invention first requires that atreatment area is established for accommodating the material t bedisposed of. These treatment areas usually consist of a flat excavatedpit next to which there are dykes banked up with the soil which has beenexcavated. To avoid contamination of the subsoil of the treatment areaand, if appropriate, also of the groundwater, it is necessary to sealoff the subsoil. For sealing, films, for example plastic films, aregenerally employed and then covered with about 10 cm of sand, it beingnecessary to provide suitable measures for checking the tightness of theindividual webs of film in the area where they overlap. For example, itis possible to introduce copper wires in the areas where the webs ofplastic overlap, and the tightness can be checked after the individualwebs have been welded together, via inductive measurements. It ispreferred to further apply a protective nonwoven to the sealing webs.

In a preferred embodiment, the protective nonwoven is further providedwith a layer of gravel incorporating drainage and a filter nonwoven as acover. This arrangement has the advantage that, on the one hand, anywater which occurs, such as, for example, percolating water, water fromprecipitation and treatment water, can be collected and fed to atreatment plant, on the other hand, since a drainage is installed, it isalso possible to combine the microbiological decontamination with theextraction method. For details of this method, see pages 11 to 12 ofthis description.

An important factor in the microbiological decontamination of soil isthe adjustment of optimum environmental factors for the microorganisms,which also includes a sufficient supply of the microorganisms withoxygen. Apart from the possibility of employing chemical oxygen donors,it is also possible to employ atmospheric oxygen as the most inexpensiveand natural microbial Oxidant. To ensure a sufficient supply of oxygen,the treatment area is therefore preferably additionally provided with anaeration system. Possibilities in this context are both passive aerationby means of drainage pipes or active aeration, or a combination ofactive and passive aeration. In the case of active aeration, filtertubes are built in above the filter nonwoven or the protective nonwoven,if no drainage had been provided. The diameters of suitable filter tubesare in general between 25 and 300 mm. The distance of the tubes to eachother is between 1 and 10 m. The individual tubes are connected to acollector line at which there is arranged a compressor. Preferably,sluice valves which allow specific aeration of individual clamps, areadditionally arranged in the individual connection lines of the filterpipes. In addition to guaranteeing optimum aeration, the arrangement ofsuch a filter tube system also has the advantage that this tube systemcan also be employed for pumping away soil air by simply exchanging thecompressor for an appropriate pumping device. Such a combination ofpumping of soil air and microbiological degradation is expedient inparticular when highly volatile contaminants are present besides otherpollutants.

As is the case in all so-called on-site methods, the method according tothe invention provides that the contaminated soil is first excavated andtreated for the elimination process. According to the invention, thistreatment consists of mixing the contaminated soil with concrete and/orbuilding rubble which, preferably, is likewise contaminated. Theconcrete and/or building rubble has previously been comminuted to aparticle size of≦10 mm, preferably≦6 mm, using suitable crushing plant.The mixing ratio of soil to concrete and/or building rubble is ingeneral between 1:9 parts by volume and 9:1 parts by volume, preferablybetween 1:1 and 3:1 parts by volume, very particularly preferablybetween 1.5:1 and 2:1 parts by volume. The optimum mixing ratio for thespecific conditions prevailing can be determined without difficulty byanyone skilled in the art with the aid of a few routine experiments.

The addition of concrete and/or building rubble facilitates adsorptionof the microorganisms and s results in their immobilization. This inturn leads to a higher multiplication rate of the microorganisms, whichmeans that higher amounts of contaminants in the soil can be degraded.In the preferred use of specialized bacterial strains which have alreadyadapted to the contamination (for example by using a strain mixture fromthe operator's own treatment plant), these specialized strains form acolony by the addition of the concrete and/or building rubble, and,caused by absorption, they are then available for a substantial time inthe soil for the elimination process. Because of the immobilization,this specialized strain can dominate over the usually present soilmicroflora for a substantial time. In addition, the addition of likewisecontaminated concrete and/or building rubble has the advantage that thismaterial is likewise eliminated and does not have to be deposited inspecial tips, as usual. The addition of comminuted concrete and/orbuilding rubble to very fine material whose permeability of air andwater is only very low, such as, for example, silty clay material,improves the permeability of air and water, and the tendency which isoften observed in such materials, to cake after prolonged, intensiveprecipitations, is drastically reduced. Finally, this treatment of thecontaminated material results in a leveling effect, i.e. in a reductionof contamination peaks, since usually the various contaminants aredistributed unevenly in the soil and concrete or building rubble.

The contaminated soil which has been treated in this manner is nowbanked up in the treatment area in the form of regeneration clamps. Theheight of these clamps is usually not more than 2 m.

The actual treatment consists of the activation of the microbiologicaldegradation by adjusting suitable environmental factors. This includesthe supply of the microorganisms with oxygen, which has already beenexplained in the description of the treatment area on pages 6 and 7 ofthis description, for example by active and/or passive aeration.Furthermore, care must be taken to provide a suitable distribution ofmoisture. It is therefore preferred to irrigate the clamps, theadditional amount of moisture ideally being adjusted as a function ofthe weather (for example amount of precipitation, ground frost etc.). Itis also preferred to monitor the addition of nutrients, such asphosphate and ammonium, together with the irrigation. Finally,atmospheric oxygen is also passed in by the irrigation. However, it isalso possible to add an oxygen donor to the treatment water.

In many cases, the microorganisms which are required for themicrobiological degradation of the contaminants are already present inthe contaminated soil, and degradation of the contaminants does not takeplace because of a lack of suitable environmental factors. In contrast,degradation of the contaminants takes place when, for example, asufficient oxygen content and sufficient moisture have been establishedin the soil.

Besides, there is also the possibility of adding specifically culturedmicroorganisms to the soil. However, it is preferred to addmicroorganisms which are already adapted to the contaminants. A highlyadvantageous variant consists in irrigation of the treatment area withwaste water from the operator's own treatment plant. In this case, themicroorganisms are already adapted to the soil pollutants since thecontaminants are constantly fed into the treatment plant in the form ofwaste water constituents. Simultaneously, the waste waters of thetreatment plant contain sufficient growth substances, such as nitrogenand phosphate, so that it is not necessary to enrich the treatment waterwith nutrients.

Typical bacteria which are capable of, for example, breaking downaliphatic and aromatic hydrocarbons, are Pseudomonas strains, such as,for example, Pseudomonas putida, Acinetobacter, Gram-positive cocci,Gram-positive rods, particularly of the Corynebacterium and Arthrobactertype, yeasts, such as, for example, Candida species, fungi, such as, forexample, Trichoderma resei, Chaetomium, Neurospora, Cladosporium,Botrytis and Penicillium.

In the method according to the invention, the treatment time requiredfor the decontamination of soil is generally around six months, but thistime depends on many external factors. On the one hand, the nature andconcentration of the contaminants and the nature and number of themicroorganisms employed play an important role, on the other hand,parameters such as, for example, the temperature, are decisive. In spiteof this, it must be noted that by the method according to the inventionit was possible to reduce markedly the necessary treatment time,compared with conventional methods.

In addition to the microbiological degradation of contaminants which hasbeen described, it is possible to extract the contaminants by means ofsuitable solvents while utilizing drainage. In this extraction, theextractants are applied by spraying. It must be ensured by suitablemeasures, such as, for example, covering the treatment area, that noextractants can pollute the environment. The extractant is thencollected by means of the drainage and worked up.

The major amount of the extractant employed is freed from thecontaminants, for example by distillation or extraction, and can then bere-used. The amount of contaminants which has been liberated, incontrast, is eliminated for example in an incinerator or the like.

Examples of substances which are suitable for the extractionare--depending on the contaminants to be removed--volatile organicsolvents, for example alcohols, water and solutions of sequesteringagents. This combination of microbiological degradation and extractionmethod is applied especially in those cases when the material to betreated contains contaminants which cannot be broken down by microbes,or only with difficulty, for example heavy metals.

A further preferred embodiment of the method according to the inventionconsists of the combination of the microbiological degradation methodwhich has been described, with the removal of soil air by pumping whichhas also been described. This preferred embodiment is applied especiallyin the case of those contaminated soils which in addition to furthercontaminants also contain highly-volatile pollutants.

The method according to the invention is now illustrated in greaterdetail with the aid of the example below.

USE EXAMPLE

First, a microbiological degradation area is established which is sealedtightly against the subsoil, due to the immediate vicinity of protecteddrinking-water catchment zones (cf. FIG. 1).

To accommodate the soil to be treated (1), a flat pit is firstexcavated, and the excavated soil is banked up on the side in the formof dyke. After the pit has been provided with a compacted layer of sand(2) of 10 cm thickness, it is covered with a polyethylene sealing web(3) (mallet plate) which is held in place on the sides. A protectivenonwoven (4) is now laid and provided with a gravel hard core (5) (grainsize 8-32 mm) to accommodate the drainage pipes. The drainage (6) opensout into a collecting shaft in which the percolating water, water fromprecipitation and treatment water which occurs can be collected and fedto the treatment plant (7). A filter nonwoven (8) is then added to thegravel layer as a cover. On top of the filter nonwoven, there isarranged an active aeration system, consisting of slotted filter tubes(φ 100 mm) which are located approximately 30 cm above the filternonwoven. The distance of the tubes to each other is 2 m. The filtertubes are connected to a collecting line via hoses and sluice valves.The valves make it possible to aerate the clamps individually. For theaeration there is also arranged a compressor with a pressure reducer.Drainage tubes (9) (φ 150 mm) are also arranged between the filter tubesfor passive aeration or ventilation of the clamps. Moreover, appropriatedevices (10) for irrigating the treatment area with clean waste waterare also arranged. The construction of the degradation area isrepresented in FIG. 1 in the form of a diagram. In what follows there isdescribed the microbiological purification of contaminated soil under anold factory building which is to be rebuilt.

The contaminated soil under the building consists of silty clay materialwith occasional sand lenses. Below a depth of about 1.5 to 2 m, the soilis dry and very hard fine silt. Such a soil is distinguished by a lowpermeability for air and water. The contaminated soil is excavated andput in intermediate storage in watertight containers.

The likewise contaminated bottom of the cellar and the contaminatedfoundations of the old building are likewise excavated and comminuted ina crushing plant down to the size of sand grains (≦6 mm).

The excavated soil shows that the distribution of the individualpollutants is highly inhomogeneous. Analyses show that there are thefollowing maximum amounts of pollutants in mg per kg of soil:

    ______________________________________                                        Benzene                       66.0                                            Toluene                       740.0                                           Ethylbenzene                  104.9                                           Xylenes                       2030.4                                          Cumene                        24.2                                            Mesitylene                    49.8                                            tert.Butylbenzene             141.1                                           Turpentine oil                                                                              hydrocarbons                                                    Phthalic esters                                                                             including polar 10200.0                                         Fatty acid esters                                                                           compounds                                                       ______________________________________                                    

The average amount of pollutants is: benzene 7 mg/kg, aromaticsubstances 171 mg/kg, hydrocarbons 432 mg/kg. No other compounds werefound.

In some cases, benzene had permeated as far as the chalk marl--about 3.5m beneath the surface--while the contamination with alkylated aromaticsubstances was nearer to the surface. The amount of phthalic esters andfatty acid esters was largely restricted to the bottom of the cellar andthe area beneath joins and cracks in the concrete. As in the case of thealkylated aromatic substances, turpentine oil was also observed near thesurface.

Microbiological examinations of the soil excavation showed that amicroflora and microfauna were present. Besides Pseudomonadaceae, mainlyfungi were found, such as, for example, Trichoderma resei, Chaetomium,Neurospora, Cladosporium, Botrytis and also Penicillium. The number ofspecimens was around 10⁴ to 9 ×10⁴ microorganisms/g of excavated soil.

The comminuted building rubble is mixed in a mixer with the contaminatedsoil in the ratio 1:1. The contents of the mixer (total contents about4000 tonnes) is transferred to the degradation area into clamps of aheight of up to 2 m.

The clamps are aerated by means of the drainage pipes (passive aeration)as well as via the aeration system (active aeration) by means of acompressor. In addition, the clamps are sprinkled with water from theoperator's own treatment plant. This irrigation is carried out as afunction of the weather (taking into account the amount ofprecipitation, danger/occurrence of ground frost). During the 6 months'treatment period from Oct. to Mar., a total of about 140 m³ of treatmentwater are applied.

In order to avoid transport into the soil to be treated of organic wastewater constituents--which have only been eliminated by sludgeadsorption--and heavy metals, the treatment was carried out using theeffluent downstream of the secondary sedimentation tank. Microbiologicalanalysis of the purified water employed showed that the followingspecies of microorganisms were present: Pseudomonadaceae, mainlyPseudomonas putida, Acinetobacter, Gram-positive cocci, Gram-positiverods, in particular Corynebacterium and Arthrobacter, as well asyeasts--Candida species. The number of specimens was around 2.2×10⁵microorganisms/g. The treated water simultaneously supplies nitrogen andphosphate as growth factors.

The use of treated water from the operator's treatment plant has theadvantage that microorganisms are added to the soil which have alreadyadapted to the pollutants of the soil, since the contaminants are fed tothe treatment plant in the form of waste-water constituents.

Microbiological examinations of the clamps showed that the species ofmicroorganisms were essentially identical to those in the effluent ofthe treatment plant. On the surfaces of the clamps, numbers ofmicroorganisms of 1.2×10⁵ to 1.8×10⁷ per g of soil were found, at thebases of the clamps, there were 2×10⁵ to mostly 2×10⁶ microorganisms/gof soil.

It can therefore be concluded that the measures resulted in greatlyincreased numbers of microorganisms, which are between 2 and 3 decimalexponents.

After a treatment time of 5 months (Oct. to Mar.) the pollutants arealready drastically reduced. Chemical analyses showed the amounts ofharmful substances listed in Tables 1 and 2. For comparison, a secondtreatment area II was established which had a structure analogous tothat of the above described area I. The difference to theabove-described area I is that this area II was only aeratedintensively, but not irrigated. After a treatment time of 5 months(autumn/winter), the concentrations of contaminants were likewiseexamined. The results are also listed in Tables 1 and 2.

A comparison of the degradation rates of the area with sprinkleirrigation with those of the area without sprinkle irrigation shows theimportant influence of the treated water which had been added.

                  TABLE 1                                                         ______________________________________                                        Maximum amount of contaminants in mg/kg of excavated soil                                                    4-5 months                                                         4-5 months Area II                                                            Area I     without                                                     Before with sprinkle                                                                            sprinkle                                                    treatment                                                                            irrigation irrigation                                     ______________________________________                                        Benzene                 66.0  0.02     0.14                                   Toluene                740.0  0.12     1.50                                   Ethylbenzene           104.9  0.05     0.19                                   Xylenes                2030.4 0.48     3.60                                   Cumene                  24.2  0.01     0.05                                   Mesitylene              49.8  0.61     1.78                                   tert.Butylbenzene      141.1  0.05     0.25                                   Turpentine oil                                                                Phthalic esters  a)    10200.0                                                                              600.0                                           Fatty acid esters                                                             ______________________________________                                         a) Summarized as hydrocarbons including polar compounds                  

                  TABLE 2                                                         ______________________________________                                        Average content of contaminants in mg/kg of excavated soil                                    4-5 months   4-5 months                                                       Area I       Area II                                                   Befopre                                                                              with sprinkle                                                                              without sprinkle                                          treatment                                                                            irrigation   irrigation                                       ______________________________________                                        Benzene     7       0.02         0.14                                         Aromatic   171      1.06         7.51                                         substances                                                                    Hydrocarbons                                                                             432      92.0         n.d.                                         including polar                                                               compounds                                                                     ______________________________________                                         n.d. not determined                                                      

The elimination sequence which has been observedtert.butylbenzene>toluene>xylenes>cumene>ethylbenzene >>mesitylene islargely identical to the information from the literature on thebiological degradation of aromatic hydrocarbons. In this context, itmust be borne in mind that in the treatment with clarified waste water amixed flora and fauna was added, and that therefore deviations frominformation from the literature, which is based on experiments withpure-grade cultures, are possible.

This sequence which has been found shows that the biological degradationis the dominant factor in the degradation of contaminants and notphysical effects, such as evaporation and/or elution by precipitationand irrigation water. If the physical effect had been dominant, thefollowing sequence would have been expected:benzene>toluene>ethylbenzene>cumene>xylenes >tert.butylbenzene>mesitylene.

What is claimed is:
 1. In a method for the microbiologicaldecontamination of soil, comprising the steps of:
 1. excavating andtreating the contaminated soil;2. banking up the treated soil on aprepared subsoil in the form of regeneration clamps; and
 3. providingthe clamps with oxygen while microbiological degradation of thecontaminants takes place;wherein the improvement comprises treating thecontaminated soil by mixing the soil with a composition selected fromthe group consisting of concrete, building rubble and mixtures thereof,which had been previously comminuted to a particle via<10 mm.
 2. Amethod as claimed in claim 1, wherein the concrete and/or buildingrubble is likewise contaminated.
 3. A method as claimed in claim 1,wherein the building rubble had previously been comminuted to a particlesize<6 mm.
 4. A method as claimed in claim 1, wherein the mixing ratiosoil : concrete and/or building rubble is between 1:9 and 9:1 parts byvolume.
 5. A method as claimed in claim 1, wherein the mixing ratio soil: concrete and/or building rubble is between 1:1 and 3:1 parts byvolume.
 6. A method as claimed in claim 1, wherein the clamps aresupplied with microorganisms during microbiological degradation of thecontaminants.
 7. A method as claimed in claim 1, wherein the clamps aresupplied with those microorganisms during microbiological degradation ofthe contaminants which are adapted to the substances contaminating thesoil.
 8. A method as claimed in claim 1, wherein the clamps aresprinkled with water during microbiological degradation of thecontaminants.
 9. A method as claimed in claim 1, wherein the clamps aresprinkled with water during microbiological degradation of thecontaminants which preferable contains microorganisms and/or nutrientsfor the microorganisms.
 10. A method as claimed in claim 1, wherein theclamps are aerated during the microbiological degradation of thecontaminants in order to provide them with oxygen.
 11. A method asclaimed in claim 1, wherein the clamps are aerated during themicrobiological degradation of the contaminants both passively by meansof drainage pipes and actively by means of a compressor and an aerationtube system.
 12. A method as claimed in claim 1, wherein the subsoil ofthe clamps is sealed by means of tight webs.
 13. A method as claimed inclaim 1, wherein a drainage is incorporated in the clamps, via which thepercolating water, water from precipitation and treatment water whichoccurs is collected and fed to a treatment plant.
 14. A method asclaimed in claim 1, wherein soil air is additionally pumped off duringthe microbiological degradation of the contaminants.
 15. A method asclaimed in claim 1, wherein an extraction of contaminants which are notbiodegradable, or only with difficulty, is additionally carried outduring the microbiological degradation of the contaminants.